Dr. H. Martin Schaefer - Research group on the evolution
and function of visual signals

Schaefer lab - Research statement

My research concentrates on the interface between ecology, animal behaviour, and evolution. I have an integrative view on research, this is why the relative focus on each of these disciplines changes according to the specific hypotheses that I am addressing. This vision statement is divided into my two main foci.

Evolution of visual communication

The relative importance of adaptation vs. constraint in the evolution of information transfer is unresolved. In the last four decades an adaptionist perspective has shaped the prevailing research programme on communication, although there is ample evidence that many informative traits serve multiple functions and that most of them did not evolve primarily for their role in communication. As such, determining the relative selection coefficients of communication on a given trait is challenging but particularly important to understand the evolutionary dynamics of information transfer. Progress in this field is hampered by a traditionally narrow focus on binary sender-receiver games in communication and by a lack of a metaphor to test and visualise adaptations, preadaptations, and exaptations.

I address the overarching theme of adaptation vs. constraints by analysing selection gradients of the relationship between the perceived phenotype (colour) and fitness. Mapping the various phenotypic traits involved in communication in an adaptive landscape will allow us further to evaluate indirect selection and phenotypic integration among distinct colour traits. Moreover, our lab is exploring quantitatively how communication is affected by differences in the sensory abilities of receivers. We are particularly interested to analyse how pleiotropy affects the evolution of communication. We are trying to establish models that are widely applicable to resolve the evolutionary significance of interspecific differences among communication systems as well as the relative contribution of sexual vs. natural selection within a given communication system.

To advance communication theory beyond its current limits we are continuing to study how abiotic factors channel the evolution communication. Plants in particular show pronounced phenotypic plasticity in communicative traits according to the habitat. This is important because environmental conditions influence not only the design but also the reliability of visual communication in plant-animal mutualisms. This research direction aims to incorporate ecology into the mainly evolutionary concept of communication theory to provide a framework for the extensive variation in visual traits between and even within populations.

Fig1:
Clusia fruit from Ecuador. The unripe fruit is green and closed (right). Upon
ripening, the fruit opens displaying a conspicuous white and red signal.

Genetic diversification and speciation

Until the last decade, the debate on speciation has predominantly focussed on the geography of speciation, that is the relative probability of allopatric vs. sympatric speciation. Various models have shown that sympatric speciation is theoretically feasible, but the conditions that lead to genetic diversification in sympatry are not well resolved. The focus on the geography of speciation has obscured two more important topics, the mechanisms of selection that underlie genetic diversification and, ultimately, speciation and the feedback mechanisms between genetic diversity and behaviour.

Mechanisms of selection
Ecological speciation predicts that divergent selection based upon contrasting environments can lead to the evolution of reproductive isolation. While ecological speciation is plausible (reviewed in Schluter Science 2009), the nature of selection leading to divergence is controversial. Specifically, there is scant evidence for the two alternative forms of selection; strong selection on a single trait or multifarious selection on multiple traits (Nosil et al. Tree 2009). The key question is thus which form of selection can be strong enough to disrupt gene flow in panmictic populations leading to local adaptation and, ultimately, speciation.

Currently, studies of reproductive isolation need to overcome three fundamental challenges. First the evolution of reproductive isolation is a historical process. Identification of the underlying mechanisms occurs thus necessarily in retrospective leading to much uncertainty and debate. Second, while it is known that divergent selection can disrupt gene flow along strong environmental gradients (Seehausen et al. Nature 2008), it is unknown whether environmental gradients are commonly strong enough so that local adaptation can disrupt gene flow in panmictic populations. Third, the time scale and the sequence of events leading to reproductive isolation are rarely known. Thus, studies correlating gene flow and adaptive divergence are faced with the dilemma that it is often impossible to establish, particularly in hindsight, whether disruption of gene flow causes divergence or vice versa.

To overcome these challenges I am studying the ongoing process of divergence in sympatric populations of the blackcap. To uncover the nature of selection I analyse various phenotypic traits that contribute to ecological performance and possible sexual isolation. I am thus able to assess the contribution of diverse behavioural and ecological premating isolating barriers to the ongoing process of isolation. Importantly, the patterns of multifarious selection on multiple traits that occur in the blackcap are potentially widespread. They are likely to occur in all species with low migratory connectivity where individuals from a single breeding population migrate to different overwintering locations or where juveniles move into distinct areas before the onset of their reproductive phase (e.g., many fishes).

Fig 2
The critically endangered Pale-headed Brushfinch feeding a young cowbird.
Cowbirds are brood-parasites that severely reduce the reproduction of the
remaining brushfinch population. Controlling the cowbird population led
to a significant increase in reproduction and subsequently to an increase
in the only known brushfinch population.

Feedback genetic diversity and behaviour

In addition to genetically induced differences, cultural differences are likely to evolve when exchanges of individuals between groups are rare. Differences in individual phenotypes and cultural norms are thought to result in positive feedback mechanisms that might propel subpopulations onto distinct evolutionary pathways. The link between genetic and cultural diversity is particularly striking in animals with a cooperative breeding system because the extent and characteristics of cooperation depend critically upon the relatedness among individuals. The overarching question is thus whether increasing fragmentation affects cultural differences in the extent of cooperative breeding behaviour. Such differences are likely to be caused by reduced genetic diversity within populations owing to reduced gene flow among populations. Furthermore, fragmentation is expected to affect the likelihood and severity of inbreeding, which can be another feedback mechanism that influences the extent of cooperation. We analyse these data in the genus Pyrrhura, neotropical parakeets, that contains 7-8 restricted range species which are globally threatened.